For example, in FIG. 6 referred to in JP-A-2005-223872, there is shown a differential drive circuit wherein a driver circuit, and an emphasizer are connected to common differential output terminals. The driver circuit is made up of a bridge circuit comprised of four units of MOS transistors, and a power source voltage thereof is supplied via a source follower circuit. The emphasizer as well is made up of a bridge circuit comprised of four units of MOS transistors, and a power source current thereof is supplied by a constant current source.